Method for preparing oxides of niobium and tantalum



amounts of other oxide impurities. applicable for treating any niobiumore concentrate or United States PatentO METHOD FOR PREPARING OXIDES OFNIOBIUM AND TANTALUM Alfred Mayer, Ottawa, Ontario, Canada, assignor toQuebec Metallurgical Industries Ltd., Ottawa, Ontario, Canada NoDrawing. Original application Aug. 5, 1957, Ser.

No. 676,384, now Patent No. 2,934,426, dated Apr. 26, 1960. Divided andthis application May 16, 1958, Ser. No. 735,679

1 Claim. (Cl. 23-22) earths. The Nigerian, South African or Norwegianconcentrates usually contain a combined content of 'Nb O and Ta O of 60to 65 percent by weight together with 20 --to 30 percent'Fe O 0.5 to 8percent SnO 0.5 to 3 percent M-nO and less than 1 percent each of SiOTiO A1 Zr0 and rare earths. The British Columbia concentrate usuallycontains a combined content of Nb O and Ta O of more than 55 percent byweight, the remainder being mostly TiO with only very small Theinvention is any ferro-niobium obtained therefrom to recoversubstantially all their tantalum and niobium contents in the form ofoxides, chlorides or metals of high purity.

At present the world production capacity for niobium is extremely smalland probably is not greater than about 25,000 pounds a year. Very fewmanufacturers are in a position to supply niobium metal in lots greaterthan '50 pounds at one time and the price for high purity niobium spongemetal (over 99%) ranges from about -$55.00 to $100.00 per pound. Thissuggests that the process costs of these manufacturers are high.-

It is well known that it is difiicult to separate niobium from tantalumbecause of their chemical similarity. The failure to find a satisfactorysolution of this difficulty probably is the principal reason for thepresent high price of niobium metal and the low world productioncapacity of this metal. It has been reported that one manufacturerproduces niobium metal by a process involving direct reduction of highpurity niobium oxide with carbon under vacuum. The high purity oxide isobtained by a compounds is obtained and niobium and tantalum metals ofat least 98 percent purity can be produced. The cost of producing thisniobium metal is sufiiciently low that a price of about one half theminimum price quoted v above is warranted.

In accordance with the method of the invention the niobium concentrateis reduced to obtain a ferro-niobium metal containing substantially allthe niobium and tantalum content of the concentrate together with metalimpurities but preferably having a low content of titanium. Theferro-niobium is chlorinated to convert all the metals thereof tovolatile chlorides. The volatile chlorides are condensed to obtain twocondensates, one containing most of the chlorides of the minorimpurities and a principal condensate which is a mixture of chlorides ofniobium, tantalum and iron with a small amount of chlorides of the minorimpurities, such as titanium, tin, silicon and aluminum.

The mixture of chlorides of niobium, tantalum and iron then is subjectedto a first distillation operation. In this operation, a first condensateis collected containing the chlorides of the remaining minor impuritiestogether with a small amount of chlorides of niobium and tantalum and aprincipal condensate is obtained containing a mixture of chlorides ofniobium and tantalum. The distillation residue contains all the iron inthe form of ferric and ferrous chlorides together with a small amount ofniobium chloride which has drained back into the still pot. This residueis treated by the basic sulphate precipitation method as describedhereinafter to obtain high purity niobium oxide. The first condensatealso may be treated by the basic sulphate precipitation method torecover the niobium and tantalum contents as oxides.

The principal condensate obtained from the above distillation operationcontaining the chlorides of tantalum and niobium then is subjected to asecond distillation op eration. In this operation, a principalcondensate is obtained which is substantially pure niobium pentachlorideand a second condensate is collected which is rich in tantalum chloridebut contains some niobium chloride. The pure niobium pentachloride isconverted to pure niobium sponge metal by reaction with magnesium metal.The second condensate is redistilled and a fraction is condensed assubstantially pure tantalum pentachloride, the remainder being recycledto said second distillation operation for retreatment. The tantalumpentachloride can be converted to sponge metal by a process analogous tothe one described herein for niobium. Y

It will be noted that in each cycle of steps of the method of theinvention three principal products are recovered, namely, (1) puretantalum pentachloride which can be converted to pure tantalum metal oroxide, (2) pure niobium pentachloride which can be converted to pureniobium sponge metal or oxide and (3) a mixture of tantalum and niobiumpentachlorides which is recycled for retreatment in a subsequent cycleof steps. The latter mixture of tantalum and niobium pentachloridesusually constitutes about 15 percent of the combined tantalum andniobium content of the original concentrate and its components areseparately recovered in pure form in the subsequent cycles of steps. 1

As previously mentioned, the residue from the first distillationoperation mentioned above, which is principally iron chlorides butcontains some niobium chloride, may be treated by a method describedhereinafter to obtain high purity niobium oxide. This niobium oxide maybe reduced by the alumino-Thermit process using high purity oxides ofother metals, such as iron, nickel, cobalt, tungsten, molybdenum, copperor tin, as the reaction initiators, to obtain high purity niobiumalloys.

The following is a more complete and more detailed description of thesteps of the method of the invention:

Preparation of ferro-niobium The niobium concentrate is mixed withaluminum and, if necessary, iron oxide and reduced by the alumino-Thermit process. Addition of iron oxide is not necessary with some oreshaving a relatively high iron content. Fluxes like calcium oxide, whichhelp reduce titanium oxide, are excluded from the charge as much aspossible. The iron oxide acts as a reaction initiator and its reductionto metal serves to facilitate coalescence of the other metals produced.Usually, a charge is used containing an amount of iron oxide sufiicientto form a term-niobium having an iron content between about 25 to 35percent by Weight. The amount of aluminum used should not be greaterthan that calculated to react with the total oxygen content of thecharge less the oxygen content of titanium oxide. By so controlling thecomposition of the charge, most of the titanium oxide content of theconcentrate is not reacted and is slagged olf. Consequently, aferro-niobium is obtained having a very low titanium conte'nt regardlessof the amount of titanium oxide in the concentrate. The preparation of aferro-niobium having a low titanium content is advantageous since itgreatly reduces the amount of chlorine consumed in the subsequentchlorination step and reduces the complexity of the equipment requiredto deal with the condensation of chlorides. On cooling the reactionmass, the metal solidifies in a solid slab surrounded by slag. The slagbreaks off cleanly from the metal surface. Slag at the bottom of themetal billet usually is a smooth, fused saucer about one half to oneinch thick while the top of the metal is covered by a layer of fusedslag several inches thick. A typical ferroniobium produced from Africanore concentrate contains in percent by weight 55 niobium, 7 tantalum,27.5 iron, 3.1 aluminum, 5.4 tin, 1.1 manganese and about 0.1 titanium.A typical ferro-niobium produced from the British Columbia concentratepreviously mentioned contains in percent by weight 61 niobium, 2.2tantalum, 31.2 iron, 2.2 aluminum and 1.6 titanium.

Chlorination of ferro-niobium The ferro-niobium billet is crushed in ajaw crusher to about /5 inch chips and charged into a refractory linedshaft furnace with a gas tight shell conventionally used forchlorination. The charge is preheated electrically to about 500 C. andthen chlorine is passed in from below the bed. The chlorination reactionis exothermic and can be controlled by the rate of chlorine addition. Asuitable operating temperature is between about 600 C. to 1000 C. Whenthe first charge is fully chlorinated, the heat content of the furnaceusually is sufiicient to raise the temperature of the next charge to 500C. without further heating.

All the metal chlorides are volatile and no residue is left in thechlorinator. The chlorides are condensed to liquid and solid in a trainof collecting vessels and the tail gas (chlorine) is passed through adust collector before being led to a caustic soda scrubber. Thus, acondensate is collected containing all the chlorides of niobium,tantalum and iron. The chlorides of the minor impurities, such aschlorides of silicon, titanium, tin or aluminum are condensed as aseparate condensate and little of the minor impurities are condensedwith the chlorides of niobium, tantalum and iron.

First distillation The condensate from the chlorination step containingthe mixture of chlorides of niobium, tantalum and iron is subjected todistillation. For this purpose, a fractionating column six feet long,packed with Raschig rings, was used. The vessel in which these chlorideswere condensed and collected is attached to the column and used as thestill pot. The still pot is heated to the boiling point of the chloridesand the column held on total reflux for several hours until equilibriumis established. During this time most of the light fractions, i.e.,chlorides of titanium, tin, silicon and aluminum, are removed. Thedistillation then is started at a very high reflux ratio to remove theremaining traces of the above mentioned light fractions together withsmall amounts of the pentachlorides of tantalum and niobium. Thesefractions are condensed at temperatures between 200 C. and 245 C. Theymay be treated to recover the tantalum and niobium as oxides by thebasic sulphate precipitation method described hereinafter. After thesefractions have been removed, the distillation is continued until no moredistillate is obtained. During this operation the still pot temperatureis allowed to rise gradually to 310 C. During the latter period, themain distillate is condensed at a temperature between 245 C. and 254 C.and consists of a mixture of niobium and tantalum pentachlorides. Theresidue in the still pot consists of ferric and ferrous chloridestogether with a small amount of niobium pentachloride which has drainedback into the still pot from the column. This residue is treated by thebasic sulphate precipitation method described hereinafter to recoverhigh purity niobium oxide.

Second distillation The main condensate obtained from the first distillation step consisting of chlorides of niobium and tantalum is subjectedto distillation using a fractionating column similar to that used in thefirst distillation step except that it has a length of 20 feet. Thevessel in which these chlorides were collected is attached to the columnand used as the still pot. The still pot is heated to a temperaturestarting at 245 C. and rising to 258 C. A first fraction of tantalumpentachloride-rich liquid is condensed at a temperature between 238 C.and 251 C- A fraction of pure niobium pentachloride is condensed at atemperature between 251 C. and 254 C.

The tantalum pentachloride-rich liquid is re-distilled and a fraction ofpure tantalum pentachloride is condensed at a temperature between 238 C.and 239.5 C. The remaining higher boiling fraction is collected andrecycled to the second distillation step for retreatment in a subsequentcycle of operations.

Reduction of niobium pentachloride t0 niobium sponge metal The pureniobium pentachloride obtained from the previous step is reduced toniobium sponge metal by reaction with pure magnesium metal at atemperature of 800 to 900 C. For this purpose, the niobium pentachloridemay be introduced as vapor or liquid into a reactor containing slightlymore than the stoichiometric amount of magnesium. Another suitablemethod for introducing the chloride and magnesium into the reactor is toplace the chloride in pure magnesium cans and drop the cans into thereactor. When the reaction is complete, a large portion of the magnesiumchloride is removed by tapping the reactor and allowing the magnesiumchloride to drain in the molten state from the reactor 0 under an inertgas blanket. This 18 not an essential step but allows a smaller vacuumsublimation retort to be used in the subsequent step. After cooling theremainder of the reaction mass in an inert atmosphere, niobium spongemetal is obtained associated with some magnesium chloride and the excessof magnesium metal used. This crude sponge metal is now heated in aretort in vacuo at about 900 C. to cause the magnesium chloride andmagnesium metal to sublime away and condense in the cooler portions ofthe retort. After cooling, the niobium metal is obtained in the form ofpure sponge metal.

The magnesium chloride and magnesium metal may be separated from thecrude sponge metal by any other suitable means, such as by leaching witha dilute aqueous acid solution. A similar cycle of steps may be used toproduce pure tantalum sponge metal. In place of magnesium metal, thereduction may be elfected by means of other metallic reducing agents,such as sodium or calcium metals.

Recovery of pure niobium oxide from residues The distillation residuefrom the first distillation described above is treated by a novel basicsulphate precipitation method to recover pure niobium oxide. 7 Thisresidue has a typical composition comprising in percent by weight 63.5chlorine, 30.6 iron, 3.2 niobium, 0.1 tantalum to less than 0.02tantalum, and 1.4 aluminum. To 100 pounds of this residue, 10 gallons ofwater is added and the mixture is digested for 3 hours at 80 to 100 C.The mixture then is diluted with 50 gallons of water. A solution of 4.6pounds of ammonium sulphate dissolved in 2 gallons of water is added andthe mixture is heated for 2 hours at a temperature between about 85 C.and 95 C. In place of ammonium sullphate, an equivalent amount of otherwater soluble sulphate may be used, such as sulphuric acid, or sulphatesof sodium, potassium, magnesium, etc. This causes a granular niobiumhydrated basic sulphate to be precipitated. The precipitate is removedby filtration, washed with water and a solution of ammonia, dried andthen calcined to obtain pure niobium oxide.

This basic sulphate precipitation method also may be used to convertpure tantalum or niobium pentachloride to pure tantalum or niobiumoxides.

The invention is illustrated further by the following specific example.

A charge consisting of 232 pounds of African concentrate, 23 pounds ofmagnetite and 80.5 pounds of aluminum was reduced by the alumino-Thermitprocess. The concentrate contained 39.4 percent niobium and 2.9 percenttitanium by weight. The ferro-niobium recovered weighed 161 pounds andcontained in percent by weight 55 niobium, 7 tantalum, 27.5 iron, 3.1aluminum, 5.4 tin, 1.1 manganese and about 0.1 titanium. The slagproduced weighed 175 pounds and contained 1.35 percent niobium byweight.

84 pounds of the ferro-niobium in the form of chips of about inch sizewere placed in a conventional chlorinator leading to a train ofcondensing vessels and a dust collector. The charge was preheatedelectrically to about 500 C. and then chlorine Was passed in below thebed. Since the chlorination reaction is exothermic, the temperatureincreased and was maintained between about 900 C. and 1000 C. bycontrolling the rate of chlorine admission to the chlorinator. All theferroniobium was converted to volatile metal chlorides and no residuewas left in the chlorinator. A principal fraction of the volatilechlorides was condensed and collected consisting of a mixture ofchlorides of iron, niobium and tantalum and containing a small amount ofminor impurities, such as chlorides of tin, aluminum, manganese andtitanium, most of the latter being condensed and collected separate fromthe principal condensate. The principal condensate weighed 214.4 poundsand contained in percent by weight 62 niobium chloride, 31.8 ironchloride, 5.1 tantalum chloride, 0.65 aluminum chloride, 0.45 tinchloride and about 0.1 titanium chloride. The product collected in thedust catcher weighed 19.1 pounds and contained 3.6 percent by weight ofniobium from which 0.95 pound of niobium oxide was recovered by thebasic sulphate precipitation method described hereinafter.

The vessel containing the 214.4 pounds of the above principal condensatewas attached to a conventional fractionating column of six foot lengthand used as the still pot. The still pot was heated to a temperaturestarting at 250 C. and rising to 310 C. A head fraction weighing 3.65pounds was condensed at a temperature between 200 C. and 245 C. fromwhich 1.5 pounds of oxide containing 43 percent niobium and 31 percenttantalum by weight was recovered by the basic sulphate precipitationmethod described hereinafter. A main fraction weighing 134.5 pounds wascondensed at a temperature between 245 C. and 254 C. The latter fractioncontained 31.7 percent niobium and 3.8 percent tantalum by weight. Theresidue in the still pot weighed 64.7 pounds and contained 3.5 percentniobium by weight, the balance being essentially a mixture of ferric andferrous chlorides. The difference in the weights of the material chargedand the materials recovered is due principally to loss due todecomposition of ferric chloride to ferrous chloride and chlorine gas.

The vessel containing the 134.5 pounds of the main condensate from thelast distillation operation was attached to a conventional fractionatingcolumn of 20 foot length and used as the still pot for a seconddistillation. The still pot was heated to a temperature starting at 245C. and rising to 258 C. A first fraction of tantalum-rich liquid wascondensed at a temperature between 238 C. and 251 C. Thisfractionweighed 17 pounds and containedabout 15 percent niobium and 28.5per? cent tantalum by weight. A main fraction of 116.5 pounds ofsubstantially pure niobium pentachloride was condensed at a temperaturebetweeen 251 C. and 254 C. The first fraction was re-distilled and 9.4pounds of substantially pure tantalum pentachloride was collected at atemperature between 238 C. and 239.5 C. with a niobium content of 1.91percent and a tantalum content of 47.6 percent, the remainder of thefraction weighing 7.59 pounds and containing 4.6 percent tantalum and31.2 percent niobium being set aside for recycling to the abovementioned second distillation operation in a subsequent cycle ofoperations.

The temperatures given in this example are those actually obtained inoperation and are subject to calibration error. They are accurate toabout 1.5 C.

30 pounds of the pure niobium pentachloride was reduced with 9 pounds ofmagnesium metal at a temperature 800 to 900 C. to produce a reactionmixture of niobium metal, magnesium chloride and the excess magnesiummetal. The reactor was tapped to permit a portion of the moltenmagnesium chloride to drain out. After cooling in an inert atmosphere,the remainder of the reaction mass was removed and heated in a retort invacuo at a temperature of about 900 C. to cause the remainder ofmagnesium chloride and magnesium metal to sublime away and condense inthe cooler portions of the retort. The niobium sponge metal obtainedweighed 9.85 pounds and was 99 percent pure. Metal recovered ascleanings from the retort weighed 0.55 pound and was set aside forrecycling to the chlorination step.

Several batches of niobium concentrate were treated by the method of theinvention and the residue of the first distillation step of each run wasset aside. A sample of 100 pounds was taken from these accumulatedresidues and treated by the previously mentioned basic sulphateprecipitation method to recover its niobium content as oxide. Thissample analyzed in percent by weight 63.5 chlorine, 30.6 iron, 3.2niobium, less than 0.1 tantalum and 1.4 aluminum. To this 100 poundsample, 10 gallons of water was added and the mixture digested at atemperature of to 100 C. for 3 hours. The mixture then was diluted with50 gallons of water. Then, 4.6 pounds of ammonium sulphate dissolved in2 gallons of water was added and the mixture was heated at C. to C. for2 hours causing a granular niobium from the group consisting of niobium,tantalum and mixtures thereof which comprises mixing water with aproduct selected from the group consisting of niobium pentachloride,tantalum pentachloride, a mixture consisting essentially of ironchloride and niobium chloride, a mixture consisting essentially ofniobium chloride, iron chloride, tantalum chloride and aluminumchloride, and mixtures of such products in the proportion oi about 10gallons of water to each 100 pounds of said product, digesting theaqueous mixture thus formed at a temperature between about 80 C. and 100C. for about 3 hours, diluting the digested mixture with Water toincrease substantially its water content, adding to the diluted mixturean aqueous solution of ammonium sulphate and heating the mixture at atemperature between about 85 C. and 95 C. for about 2 hours therebycausing a granular hydrated basic sulphate of a metal selected from thegroup consisting of niobium, tantalum and mixtures thereof to beprecipitated, removing the precipitate by filtration, and calcining theprecipitate to obtain an oxide of the selected metal, the amount of saidammonium sulphate used being such that it contains about one mole of S0group per mole of said selected metal in said product.

ReferencesCited in the tile of this patent Mellor: ComprehensiveTreatise on Inorganic and Theoretical Chemistry, Longmans, Green andCo., N.Y., vol. 9, pps. 842, 848, 877, 881 and 919.

